Plant Hormone Signalling and How a Plant Decides
A plant has no nervous system, so every decision about when to grow, when to stop and when to drop a leaf is made by the balance between a handful of small molecules. Most of these hormones do nothing alone, and the response depends on the ratio between two of them rather than the amount of either.
What a learner can do afterwards
- Name the main plant hormone classes and the developmental decision each one dominates
- Explain a response that depends on the ratio of two hormones rather than on the level of one
- Describe how a hormone signal reaches the nucleus and changes which genes are transcribed
1 · Read
A plant has no nervous system, so every decision is made by chemistry. Light, gravity, touch, and wounding each trigger hormone signals that switch growth on or off in specific tissues. Auxin collects on the shaded side of a shoot, making those cells elongate faster, so the shoot bends toward light. Roots do the opposite and bend down with gravity. These tropisms are directional growth, not movement.
Each hormone dominates one longer term decision. Gibberellins wake seeds and stretch stems. Cytokinins drive cell division. Abscisic acid shuts stomata in drought and enforces seed dormancy. Ethylene is a gas that ripens fruit and ages leaves. A dry seed waits because abscisic acid says wait, and germinates only when water tips the balance toward gibberellins.
Most hormones do nothing alone: the response follows the ratio of two, not the level of one. In tissue culture, a cell lump grows roots when auxin dominates and shoots when cytokinin dominates, with both present throughout. Apical dominance works the same way: auxin streaming from the tip suppresses side buds, so pruning the tip lowers the ratio and the bush grows bushy. Ratios work because both pathways push opposite switches on the same genes.
A hormone binds a receptor, the signal travels to the nucleus, and transcription factors switch growth genes on or off. That chain is how a nerveless plant coordinates a whole body. When you read any plant response, name the dominant hormone, name its rival, and ask which ratio the conditions favour.
A few small molecules, read as ratios and delivered to gene switches, steer the whole plant.
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